The present invention relates to a converging optical system of an optical pickup device.
There have recently been advanced research and development for the so-called high density optical disc wherein recording density of an optical information recording medium (optical disk) has been enhanced by using a blue laser beam with a wavelength of about 400 nm, and a storage capacity has been increased accordingly.
As a standard of the high density optical disc, there are known, for example, those in which an image-side numerical aperture (NA) of an objective lens is made to be about 0.85 and a protective base board thickness is made to be about 0.1 mm and those in which NA and a protective base board thickness are controlled respectively to be about 0.65 and about 0.6 mm which are similar to those of conventional DVD (digital video disc). In the following explanation, a high density optical disc having NA of about 0.65 and a protective base board thickness of about 0.6 mm is expressed as “AOD (Advanced Optical Disc)”.
There are proposed various technologies about an optical pickup device which has compatibility for the high density optical disc stated above and an optical disc which has been used widely such as DVD (digital video disc) and CD (compact disc) (for example, see Patent Document 1).
In the optical pickup device, in this case, there is caused a problem that a position of a converging spot formed on an optical axis is deviated from an information recording surface of an optical disc, if, for example, the so-called mode hop representing instantaneous fluctuation of a wavelength of a light flux takes place when power of the light flux emitted from a light source is enhanced. Therefore, it is necessary to conduct correction (hereinafter referred to as “mode hop correction”) of lens characteristics (wavelength characteristics) for the wavelength fluctuation, by providing, for example, a diffractive structure on an optical surface of an objective lens representing a single lens.
Incidentally, the mode hop correction means correction to correct aberration in a converging spot (aberration in combination of axial chromatic aberration and spherical chromatic aberration) around the wavelength fluctuation to be the diffraction limit or less.
On AOD, in particular, there is a problem that a position of the converging spot deviates greatly, because NA is as relatively large as about 0.65 and a wavelength of a light flux is as short as about 400 nm, and thereby, an amount of fluctuation of the wavelength in the case of mode hop is great.
Further, though an objective lens is made of plastic that is light in weight, low in price and is high in processability in many cases, there is caused a problem that spherical aberration is caused by temperature rise in the direction toward excessiveness, because plastic has a special character that a refractive index is changed by temperature changes. Therefore, for the purpose of improving the characteristic (temperature characteristic) of a lens for temperature changes, there is an occasion, for example, wherein spherical aberration caused by temperature rise in the direction toward excessiveness is cancelled by providing a diffractive structure on an optical surface of an objective lens representing a single lens and by generating spherical aberration in the direction toward insufficiency with the diffractive structure.
Further, there are some cases where a wavelength of an emitted light flux for each light source is different from others because of individual differences in light sources. Therefore, the axial chromatic aberration is corrected by moving a position of the objective lens representing a single lens in the optical axis direction relatively to an optical information recording medium by the use of an actuator, and the spherical chromatic aberration is corrected by using the diffractive structure provided on an optical surface of the objective lens representing a single lens. Incidentally, for environmental changes other than instantaneous changes such as mode hop, a position of an information recording surface of the optical information recording medium in the optical axis direction is usually adjusted to the position where a wavefront aberration of a converging spot is minimum, by moving the objective lens relatively to the optical information recording medium by the use of an actuator as stated above.
In the optical pickup device having compatibility for converging two light fluxes having respectively wavelength λ1 and wavelength λ2 on optical discs each being different from the other, a diffractive structure is provided on a part of an optical surface of an objective lens representing a single lens, for example, to make the light flux with wavelength λ2 passing through the diffractive structure to be flares to make the structure prohibiting light converging on an optical disk, and thereby, to provide a function (function to restrict an aperture) that restricts a numerical aperture of the objective lens for a light flux with wavelength λ2, and the diffracted light of the order that makes the diffraction efficiency to be highest among light fluxes with wavelength λ1 and with wavelength λ2 which are subjected to diffracting effect by the diffractive structure is utilized, and thereby, light utilizing efficiency is enhanced and an amount of light that is sufficient for conducting recording and/reproducing of information is obtained.
By providing a diffractive structure on an optical surface of the objective lens representing a single lens, as has been done, as stated above, technical problems such as improvement of light utilizing efficiency, improvement of wavelength characteristics and temperature characteristics and securing of a function to restrict an aperture have been solved.
(Patent Document 1)
                TOKKAI No. 2001-93179(Problems to be Solved by the Invention)        
However, there is a problem that it is difficult for the diffractive structure provided on the objective lens representing a single lens alone to solve all of the aforementioned technical problems, because the degree of freedom for the design of the objective lens is lowered.
Further, in the Patent Document 1, there is disclosed a technology to use an objective lens unit wherein a condenser lens and a diffractive optical element are combined, and by using diffracted light of two types of light fluxes (first beam of light and second beam of light) which pass through the objective lens unit, aberrations are controlled, and thereby, an optical pickup device that is suitable for downsizing can be provided.
However, when using a plane concave lens disclosed in the Patent Document 1 as the diffractive optical element, the refracting power of the plane concave lens is weaker than that of a convex lens and refracting power of the total objective lens unit is also weak, which causes a fear that an effect to correct spherical aberration by using refracting power cannot be obtained sufficiently.